Gemstone Coloration and Dyeing – The Greens
It seems that everyone likes the greens in chemically colored gemstone. Probably this is because greens are almost universally pleasing to the eye. No doubt this accounts for the great popularity of naturally green gemstone such as jade, chrysoprase, emerald, malachite, etc.
Fortunately, there are several processes that will impart green hues to some gemstone varieties. Some of these are bluish-green and are included here rather than among The Blues.
24. Chromium Chloride
- Chromium chloride (chromic chloride), CrCl3
- This is yet another simple one solution, one step process requiring only a saturated solution of the chromium chloride. This chemical is very soluble so add warm water to the crystals sparingly. Stop where there are still some undissolved crystals on the bottom of the container (glass jar) being used. Soak the clean, dried slabs in the solution for a minimum of two weeks. Pour of f the solution and store for indefinite re-use. Rinse and dry the slabs, and they are ready for use.The color induced by this process is not one of the more universally popular, but I include it here because it does appeal to some tastes in color. At best a grayish-green develops on some of the gemstone varieties but it varies among different slabs of the same variety. Perhaps the richest coloration is produced on the troyite with white polka dot agate second. Snakeskin agate is quite variable in its response to this process; some slabs color nicely while others are too pale. There may well be some gemstone varieties not discussed in this book that will respond very well to the chromium chloride process.Other chromic salts (acetate, nitrate, sulfate) will also induce color, but the results are inferior to that from the chloride.
25. Sodium Dichromate
- Sodium dichromate, Na2Cr2O7 . 2H20
- Ammonium chloride (Sal ammoniac), NH4Cl
- Prepare a saturated solution of the sodium dichromate as proscribed for Process No. 21 above. For each gallon of this sodium dichromate solution, add one pint of ammonium chloride solution prepared at the rate of one cup of the salt dissolved in one pint of warm water. Soak the clean, dried slabs in this solution for at least two weeks. Drain off the solution, and store for re-use, indefinitely. Rinse and dry the slabs in the oven at lowest heat.This process is one of two described in this book for which strong heating is part of the procedure. Therefore, a “sand bath” and heating in the oven are required. Follow carefully the directions given.The heat in the baking process decomposes the sodium dichromate to chromic oxide (chromium sesquioxide, Cr2O3). This is a rich, green, insoluble pigment known in the paint trade as “chrome green”. It is also sometimes used by lapidaries as a polishing agent. Since it is insoluble even in solvents other than water, there is no way of impregnating gemstone with it other than indirectly as in this process.Nearly all of the eighteen varieties of gemstone described in this book respond well to this process. Granite and moabite did not. The different varieties respond quite differently no doubt reflecting physical differences in their make-up. Snakeskin agate, being homogeneous (i.e. without patterns, designs, inclusions, etc.) or essentially so, colors the same way, especially in a given slab. Quite often an attractive jade-like green results. Coconut agate on the other hand tends to take on the green but with more or less of a bluish cast as does also oolite and Mexican fortification agate.A fortunate circumstance exists in the Brazil carnelian with respect to this process. The brown and reddish-brown bands in this agate owe their color to native iron impurities. The more or less uncolored bands or fortifications are amenable to impregnation with sodium dichromate. When such slabs are then subjected to the heat treatment in this process, not only do the sodium dichromate portions turn green, but the brown portions turn more or less red! The result is often very colorful. This process is one of the best for crazy lace. The natural lacy patterns and designs are considerably accentuated by the induced coloration.
The agate in the Priday Ranch thundereggs often gives spectacular results with this process. Some of these thundereggs have an agate interior of two distinct portions, (1) an interior banded portion surrounded by, (2) a more or less translucent and homogeneous agate. The latter takes on a beautiful olive green while only some, if any, of the interior bands or layers will color. The results in such slabs is quite striking.
The agatized bog responds well to this process. As in other processes, some of the components of this highly heterogeneous gemstone will absorb the colorant chemical and others will not with more pleasing results probably than if all the components were amendable to coloration. Here again some of the brownish components become reddish during the heat treatment of this process as with Brazil carnelian and probably for the same reason.
The ammonium chloride added to the sodium dichromate solution has the function of promoting the decomposition of the dichromate to produce the chrome green. Without the ammonium chloride the color conversion sometimes is incomplete. This can be disappointing, but it is not all bad either since it sometimes results in both yellow and green blending in the slabs and the cabochons made from them.
26. Copper Chloride – Potassium Dichromate
- Copper chloride (cupric chloride), CuCl2 or CuCl2 2H2 0
- Potassium dichromate, K2Cr2O7
- Prepare a saturated solution of potassium dichromate. This will require about one-half pound of the potassium dichromate crystals to a quart of warm water. Into this solution of potassium dichromate dissolve as much copper chloride crystals (or powder as the case may be) as possible. This should take about one pound of the copper chloride crystals to a pint of the potassium dichromate solution. The results is a saturated solution of copper chloride in a saturated solution of potassium dichromate. Immerse the clean, dried slabs of whatever gemstone you wish to color in this solution one at a time and allow to soak for at least two weeks. Then pour off the solution and store for indefinite re-use. Rinse and dry the slabs, and they are ready for use.This process imparts a very nice chartreuse green to some of the gemstone varieties but on others, there are brownish overtones that may be objectionable. Usually, the process works wonders with coconut, Mexican dendritic, polka dot, snakeskin and troyite agates. The others are more or less improved. For instance, the white “moons” in moon agate take on a greenish-yellow and provide a pleasing contrast with the amethyst agate surrounding them.Theoretically, sodium dichromate should work well for this process as potassium dichromate, but this has not been my experience for the reasons that, although the coloration is as good, at the same time there seems to be a greater tendency toward the browning overtones. However, sodium dichromate does do well with copper nitrate (see Process No. 31).
27. Copper Chloride – Sodium Acetate
- Copper chloride (cupric chloride), CuCl2 or CuCl2 â¢ 2H20
- Sodium acetate, NaC2H3O2 â¢ 3H20
- Prepare a saturated solution of the copper chloride as directed for Process No. 5. Soak clean, dried slabs in this solution for two weeks or more. Pour of f the solution, and store for re-use for this and other processes calling for a saturated solution of copper chloride. Rinse and dry the slabs at room temperature. Do not use an oven unless the temperature does not exceed 125°F.
- Prepare approximately a saturated solution of the sodium acetate. It is very soluble (but also inexpensive). About one pound will be required to a pint of water. Immerse the copper chloride slabs from procedure A in this solution one at a time, and allow them to soak for at least four weeks. Pour of f the solution and store for indefinite re-use. Rinse and dry the slabs, and they are ready for use.Theoretically at least, the reaction of copper chloride with sodium acetate is to produce copper acetate. This is known as “verdigris”, a well-known pigment. Copper acetate is only slightly soluble, and attempts to achieve the desired coloration directly by soaking slabs in a solution of copper acetate are predestined to disappointment. They are much too pale, at best.This process imparts a green, greenish-blue or bluish-green coloration to the slabs, depending on the gemstone variety. It is especially good on Mexican dendritic, polka dot, snakeskin and troyite agates. On the others, the coloration is apt to be too pale.Penetration of the sodium acetate in procedure 2 is very slow. Hence, less than four weeks soaking of the slabs is likely to result in incomplete penetration. If you find this to be the case when you are rough grinding a cabochon from this process, go ahead and finish it and re-run it through the process. The duration of the soaks in copper chloride and sodium acetate may be shortened considerably. One week in copper chloride and two weeks in sodium acetate are adequate.
28. Copper chloride – Borax
- Copper chloride (cupric chloride), CuCl2 or CuCl22H20
- Borax (sodium tetraborate), Na2B4O7 10H20 (The common household borax, available at the grocery store, is entirely adequate. Be sure it is borax and not ” Boraxo ” or some product where borax is merely one of the ingredients.)
- Prepare a saturated solution of the copper chloride as prescribed for Process No. 5. The same solution is called for in several of the processes presented in this book and can be used interchangeably for all of them. Soak clean, dried slabs in this solution for a minimum of two weeks.
- Prepare a saturated solution of the borax. About one cup to a quart of water is sufficient. Soak the slabs from procedure A in this solution for at least four weeks. Pour of f the solution, and store for re-use, indefinitely. Rinse and dry the slabs.The coloration resulting from this process is much the same as that induced by the copper chloride – sodium acetate process and is offered as an alternative to that process. It is also adapted to the same gemstone varieties.
29. Nickel Nitrate
- Nickel nitrate, Na(N03)2 6H20
- This is a simple, one solution, one step process. Prepare a saturated solution of the nickel nitrate as directed for procedure A of Process No. 20. The same solution may be used for both processes. Immerse the clean, dried slabs in this solution one at a time and allow to soak two weeks or more. Then pour off the solution and store for further use. It can be re-used indefinitely. If the solution becomes less than saturated as indicated by the gradual disappearance of undissolved crystals, simply add more of the nickel nitrate salt or as much as will go into solution plus a little more. Rinse and dry the soaked slabs, and they are ready for use.Nickel nitrate imparts a pale bluish-green color to a few of the gemstone varieties that is pleasing to those who like soft, pastel colors. However, the color is much too pale to be suited to most gemstone. Only Mexican dendritic, polka dot (white opaque), snakeskin and stinking water plume agates can be recommended for this process.
30. Copper Nitrate – Sodium Nitrite
- Copper nitrate (cupric nitrate), Cu(N03)2 â¢ 3H20 Sodium nitrite (not sodium nitrate), NaNO2
- Prepare a saturated solution of the copper nitrate as prescribed for Process No. 1. Soak the clean, dried slabs in this solution for at least two weeks. Remove the slabs, rinse and dry. Store the solution for indefinite re-use. This same solution can be used for other processes calling for soaking in copper nitrate. Keep the solution saturated by adding more of the copper nitrate salt from time to time to keep some undissolved crystals in the solution.
- Prepare approximately a saturated solution of the sodium nitrite. This will require about one pound of the chemical to a quart of water. Immerse the copper nitrate slabs from procedure A one at a time and allow to soak for two weeks or more. Pour off the solution and store for re-use. Rinse and dry the slabs; they are ready to use.Almost all of the eighteen gemstone varieties color well with this process, some strikingly so. The most outstanding perhaps, is snakeskin agate which takes on a beautiful, vivid green. However, Mexican dendritic, Sheep Creek dendritic, Mexican fortification, moon, polka dot (white), stinking water plume and troyite agates also respond very well. Some oolite (the more highly agatized) and some wascoite (also the more highly agatized) color nicely with this process. Bog, crazy lace and granite are the only gemstone varieties that have been disappointing.Copper chloride (cupric chloride) can be used in place of the copper nitrate in this process and theoretically, with the same results. However, I personally prefer the nitrate because it seems to give a “greener” green, and it is more tumble fast than in the case of the chloride
31. Copper Nitrate – Sodium Dichromate
- Copper nitrate (cupric nitrate), Cu(N03)2 . 3H20
- Sodium dichromate, Na2Cr2O7 . 2H20
- Prepare a saturated solution of the copper nitrate as prescribed in Process No. 1. Into this solution, dissolve sodium dichromate at the rate of one level teaspoon of the crystals in one pint of solution. Stir well. Immerse clean, dried slabs in this mixed solution one at a time and allow to soak for at least two weeks. Pour off the solution, and store for re-use, indefinitely. Rinse and dry the slabs; they are ready for use.This process is akin to Process No. 26 in that both combine a copper salt with a dichromate. However, Process No. 26 is more a yellowish-green than is produced with the present process. The latter process is very good with the same gemstone varieties as is true of Process No. 31 but especially Mexican dendritic, snakeskin and troyite agates.The amount of sodium dichromate dissolved in the copper nitrate solution can be varied to suit the taste (color taste, that is).
32. Chromium Trioxide – Copper Nitrate
- Chromium trioxide (“chromic acid”), Cr03
- Copper nitrate (cupric nitrate), Cu(N03)2 . 3H20
- Prepare a strong solution of the chromium trioxide as directed for Process No. 17. Prepare a saturated solution of the copper nitrate as for Process No. 1. Combine and mix well one part of the chromium trioxide solution with twelve parts of the copper nitrate solution. Into this mixture, immerse clean, dried slabs and allow to soak for at least two weeks. Pour of f the solution, and store for further use. Rinse and dry the slabs.The chromium trioxide-copper nitrate process imparts a color similar to that of the preceding process but perhaps somewhat paler. Actually, Process Nos. 26, 31 and 32 are similar enough to be considered more or less alternative processes. Accordingly, I have not used the chromium trioxide-copper nitrate process on all eighteen of the gemstone varieties, but it works well with Mexican dendritic, snakeskin and troyite agates, at least.
33. Cobalt Chloride – Sodium Nitrite
- Cobalt chloride (cobaltous chloride), CoCl2 6H20
- Sodium nitrite, NaNO2
- Prepare a strong solution of cobalt chloride as directed for process No. 7. Soak clean, dried slabs in this solution for at least two weeks. Pour of f the solution and store for re-use. Rinse and dry the slabs.
- Prepare a strong solution of sodium nitrite as directed for process No. 30. Soak the cobalt chloride slabs from procedure A in this solution for three weeks or more. Pour of f the solution, and store for re-use. Rinse and dry the slabs; they are ready for use.
This process is “tricky”. That is, it does not consistently produce the same results each time. It produces a deep green unlike any I have been able to produce with any other process, but sometimes it also has brownish or reddish overtones. I have not yet used this process on other than my standard test agate, snakeskin. I include it here because of the unusual green it produces. Since the cobalt chloride and sodium nitrite solutions both are called for in other processes, you should try this process incidental to those other processes.
Gemstone Coloration and Dyeing – Table of Contents
By George W. Fischer
Copyright © George W. Fischer 1990
1961 Edition, published by Lapidary Journal. Inc. San Diego, California.
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